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Subject: Re: [PATCH 3/4] dma-buf: add support for mapping with dma mapping
 attributes
To:     Liam Mark <lmark@codeaurora.org>
CC:     Christoph Hellwig <hch@infradead.org>,
        Laura Abbott <labbott@redhat.com>, <sumit.semwal@linaro.org>,
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        <john.stultz@linaro.org>
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From:   "Andrew F. Davis" <afd@ti.com>
Message-ID: <4925c9db-fc73-1ccb-1766-ef68d014d55a@ti.com>
Date:   Mon, 21 Jan 2019 14:24:09 -0600
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On 1/21/19 2:20 PM, Liam Mark wrote:
> On Mon, 21 Jan 2019, Andrew F. Davis wrote:
> 
>> On 1/21/19 1:44 PM, Liam Mark wrote:
>>> On Mon, 21 Jan 2019, Christoph Hellwig wrote:
>>>
>>>> On Sat, Jan 19, 2019 at 08:50:41AM -0800, Laura Abbott wrote:
>>>>>> And who is going to decide which ones to pass?  And who documents
>>>>>> which ones are safe?
>>>>>>
>>>>>> I'd much rather have explicit, well documented dma-buf flags that
>>>>>> might get translated to the DMA API flags, which are not error checked,
>>>>>> not very well documented and way to easy to get wrong.
>>>>>>
>>>>>
>>>>> I'm not sure having flags in dma-buf really solves anything
>>>>> given drivers can use the attributes directly with dma_map
>>>>> anyway, which is what we're looking to do. The intention
>>>>> is for the driver creating the dma_buf attachment to have
>>>>> the knowledge of which flags to use.
>>>>
>>>> Well, there are very few flags that you can simply use for all calls of
>>>> dma_map*.  And given how badly these flags are defined I just don't want
>>>> people to add more places where they indirectly use these flags, as
>>>> it will be more than enough work to clean up the current mess.
>>>>
>>>> What flag(s) do you want to pass this way, btw?  Maybe that is where
>>>> the problem is.
>>>>
>>>
>>> The main use case is for allowing clients to pass in 
>>> DMA_ATTR_SKIP_CPU_SYNC in order to skip the default cache maintenance 
>>> which happens in dma_buf_map_attachment and dma_buf_unmap_attachment. In 
>>> ION the buffers aren't usually accessed from the CPU so this allows 
>>> clients to often avoid doing unnecessary cache maintenance.
>>>
>>
>> How can a client know that no CPU access has occurred that needs to be
>> flushed out?
>>
> 
> I have left this to clients, but if they own the buffer they can have the 
> knowledge as to whether CPU access is needed in that use case (example for 
> post-processing).
> 
> For example with the previous version of ION we left all decisions of 
> whether cache maintenance was required up to the client, they would use 
> the ION cache maintenance IOCTL to force cache maintenance only when it 
> was required.
> In these cases almost all of the access was being done by the device and 
> in the rare cases CPU access was required clients would initiate the 
> required cache maintenance before and after the CPU access.
> 

I think we have different definitions of "client", I'm talking about the
DMA-BUF client (the importer), that is who can set this flag. It seems
you mean the userspace application, which has no control over this flag.

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